MONOGRAPH OF THE EXISTING CBINOIDS. 



395 



does actually occur in the earlier stajjes of yolk formation, but at a later stage 

 it is masked by the great extension and thinning out of this structure at the egg 

 periphery. If the expansion of the cytoplasm during .yolk fonnation were due 

 to the equal and simultaneous expansion of all its parts, then the form and position 

 of the yolk nucleus toward the close of yolk formation shoidd be approximately 

 similar to that seen in earlier stages, and some other cause than the simple increase 

 in bulk of the cytoplasm would have to be found in order to account for the 

 peripheral position and attenuated form which this structure invariably shows 

 at this stage. But the increase in bulk of the cytoplasm — in other words, yolk 

 formation — as has already been shown, is not uniform all over the cytoplasm, but, 

 apart from its first commencement, proceeds from without inward. The increase 

 in bidk tluis caused in the peripheral layers of tlie cytoplasm, being unaccom- 

 panied by any great increase in the superficial area of the egg, since the ex- 

 pansion is confined for the present solely to the peripheral layers, results in a 

 considerable increase in thiclmess and only a slight superficial extension of this 

 portion of the yolk. It is at this stage tliat the yolk nucleus shows the increase 

 in size already referred to. At a later stage, however, when yolk formation is 

 general all over the interior of the egg, there is a very considerable increase in 

 the size of the egg as a whole, but the peripheral layers of the yolk, having finished 

 their expansion, can only keep pace with this increase in the superficial area of 

 the egg by superficial extension. In the yolk nucleus we have a region of the yolk 

 in which these changes can be actually observed, the thinning out and superficial 

 extension which this structure shows being merely that to which the whole of 

 the peripheral portion of the cj'toplasm is at this period subjected. As we should 

 expect from this process, the yolk nucleus approximately covers the same ])ro- 

 portional area of the egg surface at all stages of yolk formation. 



Thei'e only remains to be explained why the yolk nucleus should always lie 

 at the periphery of the cell at the commencement of yolk formation, for it is 

 evident that it is primarily to this position that the clianges in the form described 

 above are due. The assumption, however, that the outward migration of the yolk 

 nucleus at the commencement of yolk formation is also passive and due to general 

 changes in the surrounding cytoplasm does not offer much difficult}'. During yolk 

 formation we have a constant migration to the periphery of granules or droplets 

 arising in the bodj' of the cytoplasm. As it is impossible to assume that this 

 outward migration is due to the activity of the droplets themselves it must be 

 concluded that there are forces in operation in the egg during yolk formation 

 which tend to send formed bodies to the periphery. These forces may be of the 

 nature of centrifugal diffusion currents, or, as perhaps is more probable, are simply 

 the result of an effort on the part of the still metabolically active cytoplasm to 

 remain in touch with the nucleus. In either case the presence of the nucleolar 

 substance on the yolk-nuclear area would serve to bring this structure under the 

 influence of such forces, though the fact that the yolk nucleus is itself, in part, 

 composed of ordinary cytoplasm would considerably weaken their action. This 

 would explain why the yolk nucleus, thoiigh always more or less peripheral in 

 position during yolk formation, is never quite so; it never competes with the 



